Heavier when charged?

[LEDagent]

Hello everyone. I just got back from Sears with my dad. We brought a dead battery from to see if it can be replaced under warrenty. It turns out that it was completely drained and that the problem is our alternator.

Anyway, while they were checking the battery, they fully charged it within an hour. When we got it back...the battery was a good 10-15 pounds heavier.

Is this common knoledge? I must admit, it was the first time for me to carry a fully charged car battery.

 

[georges80]

[ QUOTE ]
LEDagent said:

Anyway, while they were checking the battery, they fully charged it within an hour. When we got it back...the battery was a good 10-15 pounds heavier.


[/ QUOTE ]

Yep - it's the weight of all the electrons they pumped into it to charge it up /ubbthreads/images/graemlins/wink.gif

george.

 

[gyverpete]

How can the battery be heavier? Even if the electrolyte was totally evaporated (which wouldn't happen from such a discharging) and they filled it, that would have to be over a gallon and a half to account for the weight.
How can you be sure it was actually heavier and didn't just feel that way?

 

[LEDagent]

I handed the battery to my dad after i noticed how heavy it was, and even he said that there was a difference in wieght.

 

[James S]

Is this like the puzzeler on car talk and we're supposed to figure out why it was heaver when charged?

well, the answwer is obvious /ubbthreads/images/graemlins/wink.gif Or perhaps not /ubbthreads/images/graemlins/wink.gif If your alternator was bad and cooked the battery you boiled off the water. In order to charge it they would have to add the water back in...

Can you even get car batteries that you need to add water to anymore?

 

[zmoz]

[ QUOTE ]
James S said:
Can you even get car batteries that you need to add water to anymore?

[/ QUOTE ]

I didn't know they sold any without...

 

[LEDagent]

No...most, if not all, car batteries are now completely sealed. I don't think they filled it with water either...if they did, then they have a serious warrently problem then.

 

[James S]

lol zmoz, /ubbthreads/images/graemlins/grin.gif Not WITHOUT water in them, but the ones you need to top off... I bought a new car battery about 6 months ago and I had 3 or 4 choices they were all sealed.

 

[zmoz]

I know you didn't mean without water...but I just bought a battery about 4 months ago...it has to have water put into it, and I don't remember seeing any that didn't...

 

[DHCrocks]

gell cells don't have water, also the spiral cells like optima or exide orbital don't have water, you can punch a hole in the case and it won't leak.

 

[NightShift]

My mouse feels heavier when I copy something into the clipboard /ubbthreads/images/graemlins/icon3.gif

 

[LEDagent]

Well...what i thought was a ligit and serious question actually turned out to be a stupid one.

So i'm guessin the answer is no...it doesn't get heavier?....eh i dunno. Forget it. /ubbthreads/images/graemlins/icon3.gif

 

[Albany Tom]

This is also why your car goes a little bit slower when you turn the headlights on.

Seriously, even the electron count doesn't change. A battery stores enegery chemically, in the conversion of lead and lead dioxide to lead sulphite and back. Also in the process, some sulphuric is converted to water and back.

There is no change of mass, and no change of electron count. The electrons flow is in a circuit...the same number flow in that flow out.

When a lead acid battery is overcharged, some of the water is converted, by electrolysis, into hydrogen and oxygen. This is why you sometimes may have to add water to a battery.

Anyway! Except for water which may be added, the weight doesn't change! Not even a little tiny bit... On the other hand, the battery will become slightly warmer when you charge it, and when you discharge it.

 

[LEDagent]

THANK YOU Albany Tom for a decent, straight-forward answer.

Maybe the hour spent walking the tool section at Sears while we watied for the battery to be charged and tested made us a bit tired. /ubbthreads/images/graemlins/wink.gif /ubbthreads/images/graemlins/grin.gif

 

[Albany Tom]

Ah...the last box off the moving truck always seems heaviest too.

 

[Marshall Johnson]

[ QUOTE ]
Albany Tom said:
This is also why your car goes a little bit slower when you turn the headlights on.

[/ QUOTE ]

Well there WOULD be at least some merit to this, since it puts a load on your alternator which puts more load on your engine. But not by much at all. If you've ever used a portable generator and you were using something that drew a lot of current, you'd hear the idle speed drop a bit as a load is put on the alternator.

 

[Jonathan]

In terms of batteries that you need to add water to: _Most_ (though not all) car batteries use flooded cells, where you have the electrode plates sitting in a solution. If you puncture these cells, the electrolyte will leak.

As Albany Tom mentioned, if you over charge (or simply charge properly) a lead-acid battery, you will electrolyze some water into hydrogen and oxygen. If this hydrogen and oxygen escapes, then some water will be used up, which will need to be added later.

Most modern car batteries are 'maintenance free', in that they include a bit of catalyst that recombines the hydrogen and the oxygen, condensing it out as water. For these, you don't have to add water unless something goes _very_ wrong.

In terms of mass change, the reactions in the battery do not change the chemical elements in the cell, nor do they change the number of electrons in the cell. Instead the reactions just change the arrangements. Electrons don't flow out of a battery during discharge, and into the battery during charge. Instead electrons flow out of one terminal, through the load, and into the other terminal, and the _direction_ determines charge and discharge. This means that the will be no significant mass change during charge or discharge (hmm, well I guess a zinc-air battery would get heavier during discharge).

However the different configurations of elements in the battery have different potential energy levels, and thus _must_ have slightly different masses, by the mass-energy relation. In a chemical battery, the difference would be slight; I estimate that for a car battery the mass difference between discharged and fully charged would be about 0.02 picograms, and if you can feel that weight difference I've got a job for you /ubbthreads/images/graemlins/smile.gif

-Jon

 

[James S]

[ QUOTE ]
Jonathan said:

In terms of mass change, the reactions in the battery do not change the chemical elements in the cell, nor do they change the number of electrons in the cell. Instead the reactions just change the arrangements. Electrons don't flow out of a battery during discharge, and into the battery during charge. Instead electrons flow out of one terminal, through the load, and into the other terminal, and the _direction_ determines charge and discharge. This means that the will be no significant mass change during charge or discharge (hmm, well I guess a zinc-air battery would get heavier during discharge).


[/ QUOTE ]

Unlike a capacitor which does hold on to the electrons. Therefore capacitors much get heavier when they are charged.

So, how would I go about measuring the number of electrons necessary to charge my 1F 2.5v super cap? And then translate that figure into a regular weight and not an atomic weight unit?

 

[Jonathan]

I don't know what sort of capacitors you use, but the ones that I use have _two_ electrodes /ubbthreads/images/graemlins/smile.gif One electrode gets loaded up with electrons and acquires a negative charge, the other loses electrons and acquires a positive charge. The net capacitor remains electrically neutral, so the total number of electrons stays the same.

The normal unit of electric charge that we use is the Coulomb, or amp-second. Another unit is the Faraday, which happens to be 1 mole of electrons, and about 27 amp-hours. The mass of 1 Faraday of electrons is 1/1823 grams. Note, however, that if you managed to put 1 Faraday of electrons somewhere without any positive charges to balance them out, the electrostatic forces that you'd be dealing with would totally swamp any gravitational forces.

Oh, in the first line, I was being flip, and hopefully not too disrespectful. In fact, it is possible to calculate the capacitance of a single electrode (with respect to the rest of the universe), but if you charged it up it would attract or repel everything around it, and your scale would be pretty much useless.

-Jon

 

[carbonsparky]

Car batteries are usually not completely sealed. The newer maintenance free and low maintenance batteries usually have some sort of cover you can pop off to add electrolyte to the cells. depending on the battery type, some are still shipped empty and are filled at the store before sale.
These newer batteries usually never need to be topped off like the old ones except for cases of over charge. When you are over charging you can still make a battery go dry.